This invention relates to semiconductor vapor phase growing apparatus, and more particularly to a process control system capable of readily preparing and executing a program for conveniently executing the process control of the vapor phase semiconductor growing apparatus.
As apparatus for mass producing monocrystalline semiconductors is presently used semiconductor vapor phase growing apparatus. In such growing apparatus, it is important to readily and accurately set such parameters as the flow quantities of various gases used, the temperature in a reaction furnace, etc. as process steps in the reaction furnace process.
To solve these problems I have invented a process control system for semiconductor vapor phase growing apparatus comprising a reaction furnace in which vapor phase growth is made on a substrate of silicon or like semiconductor, means for heating the substrate, a pipe line network interconnecting the reaction furnace and the sources of various gases necessary for the vapor phase growth, valve means provided for the pipe line network for admitting into the reaction furnace desired quantities of the gases and control means for controlling the valve means. The control means includes a group of process programs containing a time for designating the process of the vapor phase growth in the reaction furnace and information regarding the gases employed, their flow quantities and furnace temperature, and a system program for decoding the process program group to form control signals for the valve control means. Such a process control system was disclosed in U.S. Pat. No. 4,430,959 issued on Feb. 14, 1984, the disclosure of which is hereby incorporated by reference.
More particularly, the semiconductor vapor phase growing apparatus disclosed in this U.S. Patent comprises a plurality of reaction furnaces which are programmably controlled by a common process control system. However, in the prior art process control system, various control objects are adjusted manually. For example, at each step of forming a desired epitaxially grown layer, the size of wafer used is measured and the control object is adjusted again according to the result of measurement so that there is a large probability of erroneous adjustment of the control object, thus increasing the percentage of rejects. For the purpose of preventing manufacturing of rejects, care should be taken not to erroneously measure the wafer and adjust the control object, which increases the work load of an operator.
When preparing a desired semiconductor wafer by using a vapor phase growing apparatus of the type described above, a vapor phase grown layer having a desired thickness and resistivity can be obtained when the types and flow quantities of the gases being used, operating time and the temperature of the reaction furnace are suitably selected. For this reason, the thickness and the resistivity are most important as target values of the vapor phase grown layer thus obtained. The result of measurement of the thickness and the resistivity of the vapor phase grown layer shows that the thickness (in .mu./min.) is linearly related to the quantity (g/min.) of source gas (SiCl.sub.4, SiH.sub.2 Cl.sub.2, etc.) as shown in FIG. 1, and that the resistivity (ohm-cm) has a relation shown in FIG. 2 with respect to the logarithmic value of the quantity (cc/min.) of a dopant gas per unit time (an N type dopant gas such as B.sub.2 H.sub.6 or a P type dopant gas such as PH.sub.3). In the measurements shown in FIGS. 1 and 2, hydrogen gas H.sub.2 was used as the reference base gas at a rate of 80 l/min. Consequently, from the result of measurement, the thickness and the resistivity, the target values of the vapor phase grown layer formed on the semiconductor wafer can be defined as the functions of the quantities of the source gas and the dopant gas. Accordingly, where predetermined target values are set, the quantities of the source gas and the dopant gas can be determined so that opening, closing and degree of opening of the valve devices and the temperature in the reaction furnace which are to be process controlled can readily be calculated, and the process program can also be readily prepared.
Where a number of batch operations are made for obtaining identical wafers with the same process program by repeatedly using the same reaction furnace of the semiconductor vapor phase growing apparatus, the thickness and the resistivity of the wafer tend to vary at each batch operation as shown in FIGS. 3 and 4, for example. It is considered that such tendency is caused by the fact that the interior of the reaction furnace becomes contaminated after a number of operations over a long period. For example, such tendency occurs when a feedback control system which detects from outside the temperature in the reaction furnace is used to control the heating temperature. Such error is also caused by the fact that a vapor phase grown layer is also formed on a support (sucepta) supporting the wafer, thereby gradually changing the atmosphere surrounding the wafer. Further, such tendency does not always change linearly with increase in the number of batch operations. In other words, it was confirmed that respective vapor phase growing apparatus have their own specific characteristics. For this reason, the process control of the semiconductor vapor phase growing apparatus can be made more accurate by patternizing the tendency of parameters to change in accordance with the number of batch operations so as to correct the content of the program at each batch operation.
Furthermore, in the semiconductor vapor phase growing apparatus of the type described above wherein various gases are supplied to the reaction furnace under the control of a programmable process control, the flow control apparatus plays an important role. The flow control apparatus usually comprises sensors and valves of various types, and the flow quantities of the gases detected by the sensors are compared with set flow quantities for automatically adjusting the degree of openings of the valves. Although a highly accurate control performance is required, the valves are liable to become faulty owing to their complicated construction. Accordingly, when any one of the valves becomes faulty, the percentage of rejects increases greatly. In the past, when such a problem occurs, the abnormal condition of the valve is detected for issuing an alarm which stops the operation of the vapor phase growing apparatus thus decreasing productivity. Moreover, since repair or renewal of a faulty valve is made manually, it takes a long time before resuming the normal operation.